Polyacrylic elastomers have many properties including good heat and oil stability which make them desirable for the production of various rubberlike products such as, for example, gaskets, hoses, conveyor belts, valve seals, packings, oil seals and the like.
However, polyacrylic elastomers are difficult to vulcanize even when they contain vulcanization sites such as active halogen or epoxy groups. Generally, to achieve optimum properties the molded parts require a postcure process in an oven to complete their cure. This is due to the cure rate being too slow due to the use of conventional curing agents. If the processing temperature is increased to accelerate the cure time during molding, the elastomers cure prematurely (scorch) which is not desired for proper molding. The postcure process which is therefore necessary involves capital expense, and additional labor and energy costs.
U.S. Pat. No. 3,622,547 disclosed a vulcanizable acrylic composition having an improved cure rate. It was said to provide a vulcanizable composition containing an acrylic elastomer having active halogen groups an improved state of cure and improved aging properties. This was accomplished by providing a vulcanizable composition containing an acrylic gum and a combination of a dithiocarbamic acid derivative and trithiocyanuric acid in an amount sufficient to vulcanize the composition.
The compositions disclosed in U.S. Pat. No. 3,622,547 and other prior art publications, however, cannot very well meet the two critical needs that a non-postcure system requires, namely a fast cure rate and good scorch protection. It is well known in the art that scorch protection is probably the most important factor in elastomer production. Scorchiness leads to premature curing of the elastomer, causing poor flow of the gums from which the elastomers are produced and consequent clogging of the molding apparatus. Reducing the processing temperature and/or lowering the curative content in the gums do improve scorch protection but instead lower the cure rate to unacceptably low levels.
In recent years, the trend has focused upon the development of non-postcure curative systems. For example, these systems are disclosed in the following publications.
R.M. Montagne, "Advances in acrylic elastomer cure technology" Rubber World, Volume 199, No. 3, 20 (1988).
E. Lauretti et al., "Enichem Acrylic Elastomers", PRI Rubber Conference (1984).
L.M. Centric et al., Detroit Rubber Group Technical Meeting, Detroit, MI (Oct. 13, 1988).
It is therefore an object of the present invention to provide a curative mixture that, after compounding with an acrylic gum in the usual manner, provides good scorch protection while still providing relatively high cure rates such that postcuring is not required.